Crown-of-Thorns
Huge outbreaks of the Crown-of-Thorns starfish devour coral allowing algae to take over the reef.
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Introduction
The Crown-of-Thorns starfish (Acanthaster planci) prey on coral throughout the tropical Pacific Ocean. In low densities, this sea star promotes diversity in coral reef ecosystems, as it prefers to feed on faster growing Acropoid coral. This allows coral that mature at a slower rate space to establish and grow.
However, populations of the Crown-of-Thorns starfish periodically boom and the numbers of these animals increases dramatically. 100s of thousands of sea stars will swarm a reef and devour up to 90% of the coral there. This impacts the coral ecosystem in 3 ways:
1. Loss of coral diversity and abundance.
2. Switch to an algal dominated system.
3. Changes in the species composition of the reef.
This devastation reduces diversity as many species rely on coral to provide food and shelter. Loss of biodiversity decreases the ecosystems productivity and its resilience against further disruption. See Elements of Sustainability for more information.
Causes of Crown-of-Thorn Outbreaks
Fossil evidence suggests that Crown-of-Thorn outbreaks may have been occurring for up to 7000 years. However, this finding is still being debated and it is still unknown whether these epidemics are a natural phenomenon or human induced. Human activity is known to increase the severity of starfish booms. Human influence acts in two ways:
1. Introduction of excess nutrients in the coral reef ecosystem.
A Crown-of-Thorns sea star starts life as a free swimming larva floating in the water and eating plankton. These larvae settle and mature into starfish after 2-3 weeks of this. Excess nutrients in the water provide more food for these larvae which enables more to survive to adulthood. Fertilizer runoff and sediment from poorly planned coastal development are excellent sources of these nutrients.
2. Overfishing
Several species of fish and marine snails are known to feed on the Crown-of-Thorns sea star. These predators are also prized by people as a food source, and the shell of the snails are also valued by collectors. Overfishing of these creatures means less control of starfish populations.
Biological Control of the Crown-of-Thorn Starfish
The Crown-of-Thorns sea stars have a fierce array of venomous spines. This defense, which becomes more formidable as the starfish matures, deters most potential predators. The species that are able to prey on this starfish differ throughout its lifecycle.
• Larvae
The most effective biological control occurs during the earliest stages in the Crown-of-Thorns lifecycle, as filter feeding organisms, like sponges, bivalves, sea squirts and coral, are able to feed on its eggs and free-swimming larvae.
• Juvenile
Fireworms, Harlequin shrimp and some marine snails feed on the juvenile starfish.
• Adult
Some species of Pufferfish and Triggerfish are able to prey on the mature Crown-of-Thorns. These fish and a marine snail, called the Pacific Triton, are the only predators capable of overcoming the adult sea star’s defenses. They also take juvenile starfish.
Although not predators, some species of crab and shrimp defend their coral colony homes from attacks by individual Crown-of-Thorns.
Biological control relies on a diverse and healthy reef ecosystem to be able to better withstand Crown-of-Thorns attacks.
Human Control of the Crown-of-Thorns Starfish
Attempts to control Crown-of-Thorn outbreaks are generally labor-intensive, ineffective over a wide area, and expensive. It can be dangerous as well due to the venomous spines of this starfish.
Epidemics will typically continue until disease or starvation causes starfish populations to crash. Methods used in an attempt to control the Crown-of-Thorns include:
• Cutting them up
• Removing and Burying
• Underwater fences
• Injecting Poison
Conclusion
The Crown-of-Thorns starfish has significant influence on the coral reef ecosystem, both positive and negative. Human activity may cause, and certainly influences, the severity of Crown-of-Thorns epidemics. Steps like controlling nutrient run-off and maintaining numbers of predators are more effective than artificial control methods in reducing the impact of starfish outbreaks.
How Do We Know That?
Animal Diversity Web
Australian Institute of Marine Science
University of Salzburg
